The proposed studies focus on central nervous system (CNS) HIV infection and its consequences in treated patients with plasma viral suppression. In the first of two aims, we will use the ultrasensitive single copy assay (SCA) of viral RNA to define the presence and magnitude of cerebrospinal fluid (CSF) infection in a group of 'well-treated' patients and examine the impact of 'residual' CSF infection on CSF biomarkers of immunoactivation and neural injury and on neuropsychological test performance. We will also assess whether residual CSF infection relates to combination drug penetration characteristics. In the second aim, we will use sensitive single genome sequencing (SGS) methods and phylogenetic relationships to analyze the origins of viral escape and residual CSF HIV in treated patients. While antiretroviral therapy has had a remarkable impact on severe CNS HIV disease, persistent CNS infection in treated patients remains potentially important as the driver of chronic, continued CNS immunoactivation and injury, and as a viral reservoir that needs to be targeted by viral eradication strategies. The proposed studies will provide unique and essential information regarding the frequency, character and consequences of continued CNS HIV infection in 'well-treated' patients and also provide a foundation for eradication strategies targeting this infection. PUBLIC HEALTH RELEVANCE: While antiretroviral therapy has effectively reduced the more severe neurological complications of HIV infection, central nervous system (CNS) infection by HIV unfortunately remains important in treated patients. Patients continue to show evidence of milder, but still important, neurocognitive impairment along with immunological abnormalities and breakthrough of infection in the cerebrospinal fluid (CSF), suggesting continued infection in the nervous system, and, as treatment approaches begin to extend to strategies for viral eradication or cure, this CNS infection may prove a limiting factor. After applying very sensitive viral detection methods to CSF in order to further define the presence of HIV infection in 'well-treated' patients, the results of these studies will provide novel and important information regarding the persistence of HIV and its effects on the CNS despite treatment, as well as methods for evaluating the CNS effects of future eradication strategies.